1. Field of the Invention
The present invention relates to television tuners. More specifically, the present invention relates to a television tuner in which a local oscillation signal is prevented from going out of lock in video detection.
2. Description of the Related Art
FIG. 2 shows the configuration of a conventional television tuner. Referring to FIG. 2, a television signal is converted into an intermediate-frequency (IF) signal by a frequency converter (not shown), and the IF signal is input to a surface acoustic wave (SAW) filter 31. The SAW filter 31 has an unbalanced input and a balanced output. The IF signal having passed through the SAW filter 31 is input to an integrated circuit 32 for video detection. The integrated circuit 32 includes a video detector 32a and a voltage-controlled oscillator 32b. The integrated circuit 32 has a plurality of terminals along opposite edges thereof. An IF signal input to balanced input terminals T1 and T2 of the integrated circuit 32 is supplied to the video detector 32a via an amplifier and other internal circuitry (not shown).
The voltage-controlled oscillator 32b is used to supply a local oscillation signal for synchronous detection to the video detector 32a. The voltage-controlled oscillator 32b is connected to a tank circuit 33 provided externally to the integrated circuit 32, via the connecting terminals T3 and T4. Thus, the connecting terminals T3 and T4 are connected to the tank circuit 33 via parallel connecting lines 34 and 35. The SAW filter 31, the integrated circuit 32, and the tank circuit 33 are disposed on a circuit board (not shown), so that the connecting lines 34 and 35 are implemented by conductor patterns formed on the circuit board. The voltage-controlled oscillator 32b, connected to the tank circuit 33, oscillates at a frequency that is controlled by an automatic phase control (APC) circuit (not shown) provided in the integrated circuit 32 so that the oscillation frequency will be locked to a video carrier frequency of IF signals. A video signal and an audio signal detected are output from terminals T5 and T6, respectively.
When the voltage-controlled oscillator 32b oscillates, the connecting lines 34 and 35 connecting the tank circuit 33 with the voltage-controlled oscillator 32b generate electromagnetic fields. The electromagnetic fields involve radio waves having frequencies corresponding to IF signals, so that the radio waves readily enter the input terminals T1 and T2. Therefore, the APC circuit in the integrated circuit 32 fails to control the voltage-controlled oscillator 32b appropriately, particularly when the intensity of an electromagnetic field received is low. This causes the voltage-controlled oscillator 32b to go out of lock, and results in incorrect operation of the video detector 32a. 